According to the U.S. Census Bureau, the county has a total area of 809 square miles (2,100 km2), of which 793 square miles (2,050 km2) is land and 16 square miles (41 km2) (2.0%) is water.[4] The geographic center of Wisconsin is in Wood county, nine miles southeast of Marshfield.[5]

The flat, sandy southern third of the county was largely shaped by the last glacial advance. The ice didn't reach Wood County, but it approached from the east into Portage County and butted up against the Baraboo Hills to the south. This blocked the Wisconsin River, damming it so that it backed up, forming Glacial Lake Wisconsin, a frigid lake that stretched from the Baraboo Hills north to the sites of Babcock and Wisconsin Rapids, submerging that part of the county. This area is generally flat and marshy now because meltwater rivers from the glacier and streams from land to the north carried sand and silt out into the glacial lake, where the sediment settled beneath its still waters. After the glacial dam melted enough to drain Glacial Lake Wisconsin around 13,000 years ago, the Wisconsin River cut new channels through the lake-bottom sands in the southeast corner of the county. In a later dry period, wind blew the sand into dunes. One dune in the town of Saratoga is eight meters thick. Later still, the area became wet and peat formed in places on top of the sand.[7] The first surveyors in 1852 found a great marsh, like a Wisconsin Everglades. Here is their description of what is now Cranmoor:

This Township is very nearly all covered either with Marsh or swamp there is not to exceed in the Township two Sections of land that would admit of cultivation... Timber on Swamp Tamarack & small Spruce(?) very thick. water from 6 to 20 inches deep, the marsh is covered with a light crop of grass, water from 12 to 40 inches deep, innumerable small Islands(?) interspered over this Town, the margins of which abound with Cranberries.[10]

Looking southeast across Wood County from the Marshfield moraine at Nasonville, with Powers Bluff in the middle and a plume from a paper mill at Rapids or Nekoosa on the right, almost at the far end of the county.

The north of the county was shaped by earlier glaciers, which deposited glacial till, the basis for the heavy soil there. The Marshfield moraine in the northwest corner is probably a terminal moraine from one of these earlier glaciers, or from a series of them. Its age is unclear, but its relatively smooth surface indicates that it has eroded for a much longer time than the choppy terminal moraines left 13,000 years ago,[7] like the Perkinstown moraine near Medford.

Much of the county except for the northeast corner is underlain by a layer of Cambriansandstone, formed long before the last ice age.[11] Most of the original sandstone layer has been eroded away and the remainder is usually buried under glacial till, but it can be seen in gravel pits and a few bluffs. The Lindsey bluffs (a.k.a. the Marshfield School Forest) and Birch Bluff and South Bluff in the Town of Remington are hard spots in this sandstone which have resisted erosion.[7]

Powers Bluff is different from the sandstone bluffs, much older, with a hard core of Precambrianquartzite and a peak of chert.[7] A marker on the bluff says it is a "worn down peak of an ancient mountain range which once covered northern Wisconsin."[12]

There were 30,135 households out of which 32.20% had children under the age of 18 living with them, 56.70% were married couples living together, 8.00% had a female householder with no husband present, and 32.00% were non-families. 27.20% of all households were made up of individuals and 11.40% had someone living alone who was 65 years of age or older. The average household size was 2.47 and the average family size was 3.01.

In the county, the population was spread out with 25.70% under the age of 18, 7.70% from 18 to 24, 28.40% from 25 to 44, 22.90% from 45 to 64, and 15.30% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 96.10 males. For every 100 females age 18 and over, there were 93.00 males.

Wood County has voted Republican in presidential elections from 1940 to 1992, the only exception being Lyndon B. Johnson in 1964. In 1996, Bill Clinton snapped a 28-year Republican streak in the county, taking 45.1% of the vote compared to Bob Dole's 39%. The county became a swing county during the 2000s, giving 49 and 51 percent of the vote to George W. Bush in both elections, and 55 percent to Obama in 2008. Donald Trump garnered 56 percent of the vote in Wood County in 2016.